Method and apparatus for detecting zero rate frames in a communications system

ABSTRACT

Techniques for detecting zero rate frames in a received data transmission. A modulated signal is received and demodulated in accordance with a particular demodulation format to generate demodulated symbols. The demodulated symbols are partitioned into a number of received frames. For each received frame, a quality metric is computed and compared against a threshold value. The threshold value is selected based, in part, on the quality metrics of received frames. Based on the comparison result, the received frame is indicated as being either transmitted and received in error (i.e., erased or bad) or not transmitted at all (i.e., zero rate or empty). The quality metric can relate to an energy of a received frame, a distance between a received frame and a codeword corresponding to the received frame, or other metrics. The threshold value can be selected based on the quality metrics computed for decoded frames or received frames identified as good, and can be dynamically adjusted based on current information available at the receiver. The method is advantageously used in a CDMA communications system.

CROSS REFERENCE

1. This application is a continuation of application Ser. No.09/388,029, filed on Sep. 1, 1999, and entitled “Method and Apparatusfor Detecting Zero Rate Frames in a Communications System,” now allowed.

BACKGROUND OF THE INVENTION

2. I. Field of the Invention

3. The present invention relates to data communications. Moreparticularly, the present invention relates to novel and improved methodand apparatus for detecting zero rate frames in a data transmission.

4. II. Description of the Related Art

5. Many modern day communications systems currently exist fortransmitting data from a source device to a destination device. Amongthese systems, code division multiple access (CDMA) communicationssystems are efficient data transmission systems that employ spreadspectrum techniques to utilize an entire available signal bandwidth.CDMA systems use other techniques to further enhance system capacitywhile providing the required level of performance. Such techniquesinclude dynamic adjustment of the transmit power level and datatransmission at a variable rate.

6. In CDMA systems, communication between users is conducted via one ormore base stations. A first user on one mobile station communicates to asecond user on a second mobile station by transmitting data on a reverselink to a base station. The base station receives the data and can routethe data to another base station. The data is then transmitted on theforward link of the same base station, or a second base station, to thesecond mobile station. The forward link refers to transmission from thebase station to the mobile station, and the reverse link refers totransmission from the mobile station to the base station.

7. Data transmissions for CDMA systems occur in frames of data. Toenhance system capacity, the rate of each frame can be selected from oneof a number of possible rates (e.g., full, half, quarter, and eightrates), depending on the amount of data to be transmitted. For some CDMAsystems, transmission occurs in specified (e.g., 20 msec) timeintervals, with each interval comprising a single larger (20 msec) frameor a number of smaller (5 msec) frames. Each frame can include a datatransmission or no data transmission. A frame with no transmission iscommonly referred to as a zero rate (or empty) frame.

8. The variable and zero rate frames allow the CDMA system to increasecapacity by decreasing the transmit power level, and thus reducinginterference, when smaller amounts or no data is present fortransmission. At the receiving device, a detection scheme is necessaryto detect whether a frame was received correctly (i.e., a good frame) orreceived in error (i.e., an erased or bad frame), or whether notransmission occurred (i.e., a zero rate or empty frame). Thisinformation may be required, for example, to adjust the transmit powerlevel at the transmitting source to maintain a specified level ofperformance.

9. As can be seen, techniques that can accurately identify zero rateframes are highly desirable.

SUMMARY OF THE INVENTION

10. The present invention provides novel and improved techniques fordetecting zero rate frames in a received data transmission. Zero ratedetection can be achieved using various methods. Typically, a qualitymetric is computed for a received frame that cannot be reliably decodedand compared against a threshold value. Based on the comparison result,the received frame is indicated as being either transmitted and receivedin error (i.e., erased or bad) or not transmitted at all (i.e., zerorate or empty). In accordance with different aspects of the invention,the threshold value can be 1) selected based on the quality metricscomputed for decoded frames, 2) selected based on the quality metricscomputed for received frames identified as good, and 3) dynamicallyadjusted based on current information available at the receiver. Thesefeatures increase accuracy in identifying zero rate frames by takinginto account the operating conditions of the receiver.

11. An embodiment of the invention provides a method for identifyingzero rate frames in a received data transmission. In accordance with themethod, a modulated signal is received and demodulated in accordancewith a particular demodulation format to generate demodulated symbols.The demodulated symbols are partitioned into a number of receivedframes. For each received frame, the symbols are decoded and certaindecoding metrics (e.g., symbol error rate, CRC, and so on) are checkedto determine the success of decoding. If decoding fails, or if a ratedetermination algorithm (RDA) needs to distinguish between zero rate anderased frames, a quality metric is computed and compared against athreshold value. The threshold value is selected based, in part, on thequality metrics of received frames. A particular received frame isidentified as being a zero rate frame or not a zero rate frame based onthe comparison. The method is advantageously used in a CDMAcommunications system.

12. The quality metric can relate to an energy of a received frame, adistance between a received frame and a codeword corresponding to thereceived frame, or other metrics. The energy can be computed as a sum ofsquare symbols for the received frame. The distance can be computed bydecoding a received frame, re-encoding the decoded data (if anon-systematic code is used at the transmitting device), and performinga dot product of the received frame with the decoded or re-encodedframe. The threshold value can be selected based, in part, on thecomputed quality metrics of decoded frames identified as good frames,and can also be dynamically adjusted.

13. Another embodiment of the invention provides a receiver subsystem ina communications system. The receiver subsystem includes a demodulatorcoupled to a data processor. The demodulator receives and demodulates amodulated signal in accordance with a particular demodulation format togenerate demodulated symbols. The data processor is configured to: 1)partition the demodulated symbols into a number of received frames, 2)compute a quality metric for each received frame, 3) compare the qualitymetric for a particular received frame against a threshold value, and 4)identify the particular received frame as being a zero rate frame or nota zero rate frame based on the comparison. The threshold value isselected based, in part, on the quality metrics of received frames.

14. The data processor can include: 1) a decoder that receives anddecodes the received frames to generate decoded frames, 2) a CRC circuitthat receives and checks the decoded frames to identify good framesamong the decoded frames, 3) an encoder that receives and re-encodes thedecoded frames, or a combination thereof. The quality metric can relateto an energy, a distance, or other metrics of the received frame.

15. Yet another embodiment of the invention provides a receiversubsystem used in a CDMA communications system and operable to identifyzero rate frames in a received data transmission. The receiver subsystemincludes a demodulator, a decoder, a CRC circuit, and a metriccalculation unit. The demodulator receives and demodulates a modulatedsignal in accordance with a particular demodulation format to generatedemodulated symbols. The decoder receives the demodulated symbols as aplurality of received frames, and decodes the received frames intodecoded frames. The CRC circuit receives and checks the decoded framesto identify good frames among the decoded frames. The metric calculationunit computes a quality metric for each of the plurality of receivedframes, compares the quality metric for a particular received frameagainst a threshold value, and identifies the particular received frameas being a zero rate frame or not a zero rate frame based on thecomparison. The threshold value is selected based, in part, on thequality metrics of received frames.

BRIEF DESCRIPTION OF THE DRAWINGS

16. The features, nature, and advantages of the present invention willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

17.FIG. 1 shows a diagram of an embodiment of a communications systemthat comprises multiple cells;

18.FIG. 2 shows a block diagram of an embodiment of a portion of a basestation for generating a fundamental channel and a control channel forthe forward link transmission;

19.FIG. 3 shows a block diagram of an embodiment of a portion of amobile station for processing the fundamental and control channelsreceived on the forward link transmission;

20.FIG. 4 shows a block diagram of an embodiment of a decoding unitwithin the mobile station; and

21.FIG. 5 is a plot showing two probability density functions (PDFs) fortwo hypotheses (H₀ and H₁) of a received data frame.

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS

22.FIG. 1 shows a diagram of an embodiment of a communications system100 that comprises multiple cells 110A-110G. Each cell 110 is servicedby a corresponding base station 120. Various mobile stations 130 aredispersed throughout the communications system. In an embodiment, eachmobile station 130 communicates with one or more base stations 120 onthe forward and reverse links, depending on whether the mobile stationis in soft handoff. In FIG. 1, the solid line with the arrow indicates adata transmission from a base station to a mobile station. A broken linewith the arrow indicates that a mobile station is receiving the pilotsignal, but no data transmission, from the base station. The reverselink communication is not shown in FIG. 1 for simplicity.

23. As shown by FIG. 1, each base station can transmit data to one ormore mobile stations at any given moment. The mobile stations,especially those located near a cell boundary, can receive datatransmission and pilot signals from multiple base stations. If the pilotsignal of a particular base station is above a particular threshold, themobile station can request that base station to be added to the activeset of the mobile station. In an embodiment, each mobile station canreceive data transmission from zero or more members of the active set.

24. The present invention can be applied to code division multipleaccess (CDMA) systems, time division multiple access (TDMA) systems,frequency division multiple access (FDMA) systems, and othercommunications systems. The International Telecommunications Union (ITU)recently requested the submission of proposed methods for providing highrate data and high-quality speech services over wireless communicationchannels. The majority of the proposals operate within a code divisionmultiple access environment. For clarity, the present invention isdescribed in terms of the submission by the Telecommunications IndustryAssociation (TIA) entitled “The cdma2000 ITU-R RTT CandidateSubmission,” herein after referred to as IS-2000. However, the teachingsof the present invention are equally well suited to application to otherCDMA standards proposed to the ITU. One of the proposals was issued bythe European Telecommunications Standards Institute (ETSI), entitled“The ETSI UMTS Terrestrial Radio Access (UTRA) ITU-R RTT CandidateSubmission,” hereafter referred to as WCDMA. The contents of thesesubmissions are public record and are well known in the art.

25.FIG. 2 shows a block diagram of an embodiment of a portion of thebase station for generating a fundamental channel and a control channelfor the forward link transmission. The fundamental channel can be usedto send primary data from the base station to the mobile station. In thecase of speech transmissions, the fundamental channel carries speechdata. The control channel carries control data such as status andsignaling information to the mobile station. For clarity, the inventionis described for forward link transmissions from the base station to themobile station, but is equally applicable for reverse link transmissionsfrom the mobile station to the base station.

26. As shown in FIG. 2, a message generator 212 generates and providescontrol messages to a cyclic redundancy check (CRC) and tail bitgenerator 214. Generator 214 appends a set of CRC bits used to check theaccuracy of the decoding at the mobile station. The CRC bits are paritybits generated based on the contents of the particular control message.Generator 214 further appends a set of tail bits to the control messageto clear the memory of the decoder at the mobile station. The formattedcontrol message is then provided to an encoder 216 that encodes themessage with a particular encoding format. Encoder 216 provides forwarderror correction (FEC) coding of the control message. In a specificembodiment, encoder 216 is a rate one-half or a rate one-quarterconvolutional encoder, as defined by the IS-2000 submission. The encodedsymbols from encoder 216 are provided to a symbol puncturer 220 thatpunctures, or removes, some of the symbols in accordance with aparticular puncturing pattern. The unpunctured symbols are provided toan interleaver 222 that reorders the symbols in accordance with aparticular interleaving format. The interleaved symbols are provided toa modulator 230.

27. A variable rate data source 232 generates variable rate data. Thedata can comprise speech, video, facsimile, multimedia, electronic mailmessages, and other forms of digital data. An example of a method fortransmitting data in code channel frames of fixed duration is describedin U.S. Pat. No. 5,504,773, entitled “METHOD AND APPARATUS FOR THEFORMATTING OF DATA FOR TRANSMISSION,” assigned to the assignee of thepresent invention, and incorporated herein by reference. Generally,variable rate data source 232 can support any number of rates, and canalso support zero rate for no data transmission.

28. In a specific embodiment, variable rate data source 232 is avariable rate speech encoder such as the one described in U.S. Pat. No.5,414,796, entitled “VARIABLE RATE VOCODER,” assigned to the assignee ofthe present invention, and incorporated herein by reference. Variablerate speech encoders are popular in wireless communications becausetheir use increases the battery life of wireless communication devicesand enhances system capacity with minimal impact on perceived speechquality. The Telecommunications Industry Association has codified somepopular variable rate speech encoders in such standards as InterimStandard IS-96 and Interim Standard IS-733. These variable rate speechencoders encode the speech signal at four possible rates based on thelevel of voice activity. These rates are referred to as full rate, halfrate, quarter rate, and eighth rate. Each rate is associated with aparticular number of bits used to encode a frame of speech, with thefull, half, quarter, and eight rates respectively using one, one-half,one-quarter, and one-eight a specified maximum number of bits to encodethe frame. The rate can vary on a frame-by-frame basis.

29. Variable rate date source 232 provides the data, in frames, to a CRCand tail bit generator 234. Generator 234 appends a set of CRC bits usedto check the accuracy of the decoding at the mobile station. Again, theCRC bits are parity bits generated based on the contents of theparticular data frame. Generator 234 also appends a set of tail bits tothe data frame to clear the memory of the decoder at the mobile station.The formatted frame is then provided to an encoder 236 that encodes theframe with a particular encoding format. Encoder 236 provides forwarderror correction coding of the data. In a specific embodiment, encoder236 is a convolutional or a turbo encoder operated at either rateone-half or rate one-quarter, as defined by the IS-2000 submission. Theencoded symbols from encoder 236 are provided to a symbol repetitiongenerator 238 that repeats the encoded symbols of lower rate frames. Thesymbols are then provided to a puncturing element 240 that puncturessome of the symbols in accordance with a particular puncturing patternto provide a particular number of symbols for each frame. Theunpunctured symbols are provided to an interleaver 242 that reorders thesymbols in accordance with a particular interleaving format. Theinterleaved symbols are provided to modulator 230.

30. In an embodiment, modulator 230 modulates the fundamental andcontrol channels in accordance with a particular CDMA modulation formatand provides a modulated signal to a transmitter (TMTR) 252. Forexample, modulator 230 can scramble the data with a long PN sequence,spectrally spread the data with short PN sequences, cover the data withWalsh codes, and quadrature modulates the data with an inphase and aquadrature carrier signal. Transmitter 252 amplifies, filters, andupconverts the signal. The forward link signal is then provided througha duplexer 254 and transmitted from an antenna 256. The elements shownin FIG. 2 are described in further detail in the IS-2000.

31.FIG. 2 shows a simplified block diagram of the fundamental andcontrol channels. Other channels are also available for datatransmission on the forward link but not shown in FIG. 2 for simplicity.

32.FIG. 3 shows a block diagram of an embodiment of a portion of themobile station for processing the fundamental and control channelsreceived on the forward link. The forward link signal from the basestation is received by an antenna 312, routed through a duplexer 314,and provided to a receiver (RCVR) 316. Receiver 316 downconverts thereceived signal to a baseband frequency in accordance with ademodulation format that is complementary to the modulation format(e.g., QPSK) used at the base station. The baseband signal is thenprovided to, and demodulated by, a demodulator (DEMOD) 318 to providedemodulated symbols. Demodulator 318 performs functions complementary tothose performed at the base station (e.g., decovering, despreading, anddescrambling). The demodulated symbols are provided to a de-interleaver(DEINT) 320 that reorders the symbols in accordance with ade-interleaving format that is complementary to the interleaving formatused at the base station. The reordered symbols are provided to adecoding unit 322 that decodes the symbols to provide an estimate of thetransmitted frame. Using the CRC bits, if any, included in thetransmitted frame, the estimate of the transmitted frame is then checkedto determine the accuracy of the frame estimate. The decoded data isprovided to a processor 330.

33. In an embodiment, the mobile station performs a blind decoding onthe forward link signal. Blind decoding describes a method of decodingvariable rate data in which the receiver does not know a priori the rateof the data transmission. In an embodiment, the mobile stationdeinterleaves, accumulates, and decodes the data in accordance with eachpossible rate hypothesis (e.g., full, half, quarter, eight, and zerorates and erasure). One of the decoded frames is selected as the bestestimate based on one or more quality metrics such as the symbol errorrate, the CRC check, the Yamamoto metric, the frame energy, and othermetrics.

34.FIG. 3 also shows some of the circuit elements used to transmiterasure indicator bits (EIBs) to the base station for forward link powercontrol. In an embodiment, the EIBs are multiplexed with the reversetraffic data and provided to a modulator (MOD) 332 that combines theEIBs with the traffic data at particular locations defined by theIS-2000 submission. The combined EIBs and traffic data are modulated bymodulator 332 using a particular modulation format. The modulated datais provided to a transmitter (TMTR) 334 that upconverts, amplifies, andfilters the signal prior to transmission to the base station via antenna312. In an embodiment, the reverse link signal is a CDMA signal that ismodulated in accordance with the IS-2000 submission.

35. To enhance system capacity, the CDMA system is designed to transmitdata using frames of various frame formats and rates. Each frame formatcan be defined by a particular frame length, a particular coding format,and (possibly) some other attributes. For example, in accordance withthe IS-2000 submission, data is transmitted in 5 msec or 20•L msecframes, where L is 1, 2, or 4. The rate of each 20•L msec frame can alsobe selected from one of a number of possible rates (e.g., eight or morerates), depending on the amount of data to be transmitted and otherconsiderations. For an IS-2000 compliant system, transmission occurs in20 msec intervals, with each interval comprising one 20 msec frame, four5 msec frames, or a portion of a longer frame. Each frame can include adata transmission or no transmission. The 5 msec frame has lessprocessing delay, and is particularly useful for transmitting controlmessages that need to be acted on quickly. As currently specified by theIS-2000 submission, a zero rate frame can be transmitted on a 5 msecframe or a 20 msec frame on the dedicated control channel (e.g., whenthere are no control messages to send), and a zero rate frame may betransmitted on the fundamental channel when the transmitter is out ofpower. A zero rate frame may also be transmitted on a particular (e.g.,supplemental) channel if there is no information (e.g., no voice data)to send.

36.FIG. 4 shows a block diagram of an embodiment of decoding unit 322.The demodulated data from de-interleaver 320 is provided to a number offrame decoders 410A through 410N. Each frame decoder 410 can be used todecode a data frame based on a particular decoding hypothesis (i.e., aparticular frame format and rate). A data processor can be designed toinclude all elements, additional elements, and/or a subset of theelements in frame decoder 410.

37. On the forward link, data transmission at lower rates is achieved byrepeating each code symbol N times (where N is 1, 2, 4, or 8) to achievea particular symbol rate. Each transmitted symbol is also scaled by 1/Nto provide approximately the same amount of energy per code symbol. Atthe receiver, each set of N repeated symbols are accumulated and scaledto provide a combined soft decision symbol that is representative of theoriginal code symbol.

38. Within each frame decoder 410, the demodulated data is provided to asymbol accumulator 412 that accumulates sets of N received symbols basedon a hypothesized rate of 1/N. For example, if the frame decoder isconfigured to decode an eight rate frame, symbol accumulator 412accumulates sets of eight received symbols to generate a soft decisionsymbol for each set. Each soft decision symbol is representative of theoriginal symbol at the transmitting device. The soft decision symbolsare provided to a decoder 414 that decodes the symbols to providedecoded data. Decoder 414 is designed based on the encoder used at thetransmitting source. For example, a Viterbi decoder is preferably usedto decode convolutionally encoded data. Decoder 414 or other externalcircuits may further be designed to provide a frame quality metric suchas a symbol error rate, a CRC check, a Yamamoto quality metric, or acombination thereof, that can be used to determine the quality of thedecoded frame. The Yamamoto quality metric is particularly useful forlower rates when CRC bits are not available.

39. An efficient decoding scheme for data is disclosed in U.S. Pat. No.5,933,462, entitled “SOFT DECISION OUTPUT DECODER FOR DECODINGCONVOLUTIONALLY ENCODED CODEWORDS,” and U.S. Pat. No. 5,710,784,entitled “MULTIRATE SERIAL VITERBI DECODER FOR CODE DIVISION MULTIPLEACCESS SYSTEMS APPLICATIONS,” both assigned to the assignee of thepresent invention, and incorporated herein by reference.

40. For some rate hypotheses that include CRC bits, the decoded data isprovided to a CRC circuit 416 that checks the CRC bits appended witheach decoded frame. CRC check is known in the art and further defined bythe particular CDMA standard being implemented (e.g., IS-95-A orIS-2000). In an embodiment, CRC circuit 416 provides a one-bit resultfor each checked frame. In a specific implementation, the CRC result isa logic zero (“0”) if the CRC check indicates a good frame and a logicone (“1”) if the CRC check indicates a frame that is not good (i.e.,erased or empty).

41. Decoding unit 322 can be designed in various configurations. Forexample, for an IS-2000 compliant system, decoding unit 322 can includea number of frame decoders 410 operated in parallel, with each framedecoder 410 configured to decode a particular decoding hypothesis. Therate determination can be performed based on the symbol error rate, theCRC result, the Yamamoto quality metrics, other metrics, or acombination thereof. One such decoder design is disclosed in U.S. Pat.No. 5,774,496, entitled “METHOD AND APPARATUS FOR DETERMINING DATA RATEOF TRANSMITTED VARIABLE RATE DATA IN A COMMUNICATIONS RECEIVER,”assigned to assigned to the assignee of the present invention, andincorporated herein by reference.

42. In FIG. 4, for ease of understanding, decoding unit 322 is shown ashaving multiple parallel paths for processing the demodulated symbols.However, a single decoding path using shared circuit elements ispreferred in some implementations to reduce the amount of requiredcircuitry. In the shared decoder implementations, the demodulatedsymbols are stored in a buffer (not shown in FIG. 4) as they arereceived and repeatedly provided to a frame decoder for decoding. Theframe decoder is reconfigured for a different decoding hypothesis foreach pass of the data. Other implementations of decoding unit 322 can becontemplated and are within the scope of the invention.

43. Detection of a zero rate frame may be required for manyapplications. In an IS-2000 system, a power control mechanism isprovided to adjust the transmit power of the forward link signal basedon the decoded forward link frames at the mobile station. The mobilestation decodes the forward link frames and determines whether theframes are good, erased, or not transmitted. The base station isinstructed to adjust the forward link transmit power level based on thedecoded frames. For example, the base station can be instructed todecrease its transmit power to the mobile station if a decoded frame isgood, increase the transmit power if the decoded frame is bad (orerased), and do nothing if no transmission (or zero rate) is detected.The quality of the communication and the capacity of the system aredependent, in part, on the ability to accurately detect erased and zerorate frames.

44. IS-2000 defines a power control mechanism for the forward link.Specifically, when operating in certain specified modes, the mobilestation is required to set all power control bits on a Reverse PowerControl Subchannel during a 20 msec period to an EIB, which is definedby the following:

45. 1) The EIB bit is set to “0” in the second transmitted framefollowing the detection of a good 20 msec frame on the ForwardFundamental Channel or the Forward Dedicated Control Channel.

46. 2) The EIB is set to “0” in the second transmitted frame followingthe detection of at least one good 5 msec frame without detection of anybad (i.e., erased) 5 msec frame.

47. 3) The EIB is set to “1” in the second transmitted frame for allother cases.

48. The IS-2000 specification is tabulated in Table 1 for variousdecoding scenarios. TABLE 1 Decoder and Zero Rate Detector Output 1^(ST)2^(ND) 3^(RD) 4^(TH) Scenario 5 msec 5 msec 5 msec 5 msec 20 msec EIB 1Bad Bad Bad Bad Good 0 2 Good Empty Empty Empty Bad/Empty 0 3 Empty GoodEmpty Empty Bad/Empty 0 4 Empty Empty Good Empty Bad/Empty 0 5 EmptyEmpty Empty Good Bad/Empty 0 6 Good Good Empty Empty Bad/Empty 0 7 GoodEmpty Good Empty Bad/Empty 0 8 Good Empty Empty Good Bad/Empty 0 9 EmptyGood Good Empty Bad/Fmpty 0 10 Empty Good Empty Good Bad/Empty 0 11Empty Empty Good Good Bad/Empty 0 12 Good Good Good Empty Bad/Empty 0 13Good Good Empty Good Bad/Empty 0 14 Good Empty Good Good Bad/Empty 0 15Empty Good Good Good Bad/Empty 0 16 Good Good Good Good Bad/Empty 0 17Bad 5 msec frame(s) anywhere Bad/Empty 1 18 Empty Empty Empty EmptyEmpty 1

49. As shown in Table 1, the EIB is set to logic low if: 1) the received20 msec frame is decoded as a good frame, or 2) at least one received 5msec frame within a 20 msec time interval is decoded as a good frame ANDa bad (i.e., erased) frame is not detected. A frame can be identified asbeing good by performing a CRC check on the decoded frame. For thesecond case in which at least one 5 msec frame is detected as beinggood, the remaining 5 msec frames in the 20 msec time interval need tobe identified as being either bad or empty. Zero rate detection is thusneeded for this case.

50. Referring to Table 1, when the EIB is set to zero, the decoder hasinformation from at least one good frame. In accordance with an aspectof the invention, for improved detection accuracy, the information fromgood frames can be used to assist in determining whether a decoded frameis bad or empty.

51. Zero rate detection can be achieved using various methods.Generally, a quality metric is computed for a received frame andcompared against a threshold value. Based on the comparison result, thereceived frame is indicated as being either transmitted and received inerror (i.e., erased or bad) or not transmitted at all (i.e., zero rateor empty). In accordance with an aspect of the invention, the thresholdvalue can be selected based on the quality metrics computed for received(and possibly decoded) frames. In accordance with another aspect of theinvention, the threshold value can be selected based on the qualitymetrics computed for received frames identified as good. In accordancewith yet another aspect of the invention, the threshold value can bedynamically adjusted based on current information (or futureinformation, if the current decision is delayed) available at thereceiver. These features increase accuracy in identifying zero rateframes by taking into account the operating conditions of the receiver.

52. In one zero rate detection method, the sum of the squared symbols iscomputed and compared against a threshold value. The sum of the squaredsymbols is indicative of the energy of the received frame. A datatransmission is indicated if the computed energy is greater than anenergy threshold value, and no transmission is indicated if the computedenergy is less than the threshold value.

53.FIG. 4 shows a block diagram of the circuitry used to detect zerorate using the sum of the squared symbols. The soft decision symbolsfrom symbol accumulator 412 are provided to a sum of squared symbolselement 422. Element 422 squares each received soft decision symbol in aparticular frame and sums the squared symbols within the frame. The sumresult represents the computed energy for the frame and is provided toprocessor 330. In an embodiment, processor 330 considers two hypothesesfor the computed energy value, which are:

54. H₀—the computed energy contains only noise, and

55. H₁—the computed energy contains signal plus noise.

56. Specifically, processor 330 determines whether the computed energyis likely to contain only noise (i.e., hypothesis H₀) or signal plusnoise (i.e., hypothesis H₁). Based on the result of this determination,a received frame is indicated as being erased or zero rate. The zerorate determination is described in more detail below.

57. The computation of the noise and the signal plus noise of aparticular communications channel is described in further detail in U.S.Pat. No. 5,903,554, entitled “METHOD AND APPARATUS FOR MEASURING LINKQUALITY IN A SPREAD SPECTRUM COMMUNICATION SYSTEM,” assigned to theassignee of the invention, and incorporated herein by reference.

58. In a second zero rate detection method, the decoded symbols arere-encoded and correlated with the soft decision symbols. For aparticular frame, a dot product is performed between the (encoded) softdecision symbols and the re-encoded symbols. The dot product isindicative of a distance between the received vector (i.e., the receivedframe) and its nearest codeword (i.e., the re-encoded frame). Thecomputed distance is compared against a distance threshold. A datatransmission is indicated if the computed distance is less than adistance threshold value, and no transmission is indicated if thecomputed distance is greater than the threshold value.

59.FIG. 4 also shows a block diagram of the circuitry used to detectzero rate using the computed distance. The decoded bits from decoder 414are provided to an encoder 424 that encodes the bits using the sameencoding format used at the transmitting source for the particulardecoding hypothesis. For example, encoder 424 can be a convolutional orturbo encoder and can be a rate one-half or rate one-quarter encoder, asdefined by the IS-2000 submission. The code symbols from encoder 424 areprovided to a dot product element 426 that also receives the softdecision symbols from symbol accumulator 412. Dot product element 426performs a dot product of the soft decision symbols with the re-encodedsymbols in a manner known in the art, and provides the result toprocessor 330. The dot product result is indicative of the distancebetween the received and re-encoded frame. Processor 330 then considersthe two hypotheses (described above) for the computed distance.

60. In a third method for zero rate detection, which is a variation ofthe second method, the decoded data bits are correlated with the softdecision symbols. For a systematic code, the encoded data includes theoriginal data and coded (or parity) data. This property allows thedecoded data bits to be correlated with the data portion of the encodeddata (i.e., the soft decision symbols). This method eliminates the needfor re-encoding, which simplifies the decoding circuitry and shortensthe processing time to detect zero rate frame. This method is especiallyapplicable to an IS-2000 compliant system, which employs a systematicturbo code for the supplemental channel on the forward link.

61. In the description above, the quality metrics are computed for eachreceived frame. However, the quality metric can be computed for afraction of a frame or for multiple frames, and this is within the scopeof the invention.

62. For many CDMA systems, a pilot signal is transmitted on the forwardor reverse link to allow the receiving station to perform variousfunctions. As part of the signal processing, the pilot signal isrecovered and used to coherently demodulate the forward link signal.Thus, the demodulated symbol includes a factor that is related to thepilot energy.

63.FIG. 4 includes circuitry used to compute several quality metrics(e.g., energy and distance) for identifying zero rate frames. Typically,only one quality metric is computed, and the frame decoder in FIG. 4 canbe simplified. The quality metrics can also be computed in variousmanners such as by hardware specifically designed to perform thefunctions described herein, by software programmed to perform thedescribed functions, or a combination of both. For example, sum orsquared symbols element 422 can be implemented by software executed onprocessor 330. Processor 330 can be implemented in a microcontroller, amicroprocessor, a digital signal processing (DSP) chip, or anapplication specific integrated circuit (ASIC) programmed to perform thefunction as described herein.

64. As noted above, the computed quality metric for a particular frameis compared against a threshold value to determine whether a zero rateor erased frame was received. The quality metric can be the energycomputed using the first method, the distance computed using the secondand third methods, or other metrics. The energy and distance metricshave an inverse relationship. Specifically, a zero rate frame is morelikely if the computed energy is low or if the computed distance islarge. For simplicity, the following description is directed toward thecomputed energy, but may be modified to cover the computed distance orother metrics.

65.FIG. 5 is a plot showing two probability density functions (PDFs) forthe two hypotheses (H₀ and H₁) of a received frame, which are identifiedabove. A PDF 510 corresponds to hypothesis H₀ in which the computedenergy contains predominantly noise, and a PDF 512 corresponds tohypothesis H₁ in which the computed energy contains signal plus noise.PDF 510 has a mean of x₀ and a standard deviation of σ₀, and PDF 512 hasa mean of x₁ and a standard deviation of σ1. The mean of PDF 510 is lessthan the mean of PDF 512, as expected. The distance between x₁ and x₀corresponds to the mean signal energy of the received frames.

66. If PDFs 510 and 512 are known, a threshold 514 can be set at a valuex_(TH) such that a desired outcome is achieved. For example, if thedesired outcome is to obtain the same likelihood of detection error foreither hypothesis, then a threshold value x_(TH1) can be selected suchthat an area 520 to the right of x_(TH1) and under PDF 510 is equal toan area 522 to the left of x_(TH1) and under PDF 512. If the thresholdvalue is set lower than x_(TH1), toward x₀, the probability of missinghypothesis H₁ is decreased but the probability of false detection forhypothesis H₀ is increased. That is, if the threshold value is set lowerthan x_(TH1), a computed value belonging to hypothesis H₁ is more likelyto be correctly identified but a computed value belonging to hypothesisH₀ is more likely to be incorrectly identified.

67. The desired outcome may be dependent on various considerations. Forexample, if the zero rate determination is used for controlling thetransmit power of a transmitting source, it may be more desirable to errtoward transmitting more power than necessary (which may reduce systemcapacity) than to transmit less power than required (which may degradeperformance).

68. In an embodiment, PDF 510 is estimated from the computed metricsfrom empty frames and PDF 512 is estimated from the computed metricsfrom good and bad frames. As noted above, a frame can be identified asbeing correctly decoded (i.e., good) based on the result of the CRCcheck. Frames that do not pass the CRC check are identified as eitherbad or empty. In an embodiment, statistics such as the mean and standarddeviation are computed or evaluated using nominal operation conditions.Subsequently, the statistics are computed using information from bothgood and bad frames as they occur in the actual operation. The mean andstandard deviation can also be computed for the metrics associated withempty frames. Initially, statistics for empty frames can be estimatedfrom the total noise power Nt on a known channel (e.g., the pilotchannel). The estimation of Nt is disclosed in the aforementioned U.S.Pat. No. 5,903,554.

69. In an embodiment, the PDFs are assumed to be Gaussian. The shape ofa Gaussian PDF is uniquely defined for a given mean and standarddeviation. PDF 512 can be determined from the mean and standarddeviation for good and bad frames, and PDF 510 can be determined fromthe mean and standard deviation for empty frames. Based on these PDFs,the threshold value x_(TH) can be selected such that the desired outcomeis achieved, as described above.

70. For zero rate detection, the computed metric for a particular frameis compared against the threshold value x_(TH). If the computed metricis less than the threshold value x_(TH), the frame is identified as azero rate or empty frame. Otherwise, if the computed metric is greaterthan the threshold value x_(TH), the frame is identified as a bad orerased frame. It should be noted that a different selection criterion isused if the quality metric is the computed distance between received anddecoded frames.

71. The threshold value x_(TH) can be adjusted to account for additionalinformation available to the receiver. For example, since PDF 512 forhypothesis H₁ is initially determined from frames known to be good, andthat good frames tend to contain more energy than erased frames, theaverage x₁ will be slightly higher than the true mean value forhypothesis H₁. Thus, the threshold value x_(TH) can be slightly skewedor offset to the left of x₁. Viewed differently, since the computedmetric is known to be from an empty frame (i.e., no transmission) or abad frame (having low received energy, which is possibly the reason theframe is decoded as an erasure), it is likely to be a smaller value andthe threshold value should be offset to the left accordingly. The amountof offset can be determined based on system simulation, empiricalmeasurements, or by other means. For example, lab measurement can bemade on a number of transmitted frames. The (mean) difference in thecomputed metrics between good and bad frames can then be determined andstored as a parameter in the receiving device. Subsequently, thecomputed mean for good frames can be offset by this mean difference toderive an estimate of the mean for bad frames. The threshold value canthen be set based on the estimated mean for bad frames and the estimatedmean for empty frames.

72. The threshold value can also be adjusted based on other availableinformation, such as power control information. The transmit power ofthe transmitting source may be adjusted by a power control loop toprovide a particular level of performance (e.g., a particularframe-error-rate FER) at the receiving device. In one implementation,the power control loop measures the quality (e.g., the Eb/Nt) of thereceived signal, compares the measured signal quality against a setpoint, and adjusts the transmit power of the transmitting source suchthat the signal quality is maintained at the set point. The set point isadjusted such that the desire performance is achieved. In thisimplementation, the threshold value can be adjusted, for example, by thedifference between the set points for good and bad frames. For example,if the set point is 5 dB when a frame is correctly decoded and 4 dB whena frame is incorrectly decoded, the threshold value can be adjusteddownward by 1 dB.

73. The threshold value can also be adjusted based on a decisionfeedback decoding scheme. Initially, the threshold value can be set to aparticular value based on information then available, such as theinitial estimated statistics for empty frames and bad frames.Thereafter, decoding is performed iteratively whereby information fromincorrectly decoded frames is used to update the statistics for PDFs 510and 512. For example, a frame incorrectly decoded can be estimated aseither an empty frame or a bad frame, and the compute metric for thisframe is used to update the statistics for PDF 510 or 512, respectively.In this manner, the decoded data is used in decoding future data (e.g.,via the adjustment of the threshold value).

74. By setting the threshold value x_(TH) based on measurements computedat the receiver, the operating conditions of the particular receiver aretaken into consideration in making the zero rate determination. Forexample, if the receiver requires more power to maintain a particularlevel of performance, this fact will be taken into account in settingthe threshold value.

75. The threshold value can also be dynamically adjusted as operatingconditions change. The computed statistics (e.g., mean and standarddeviation) can be determined based on a weighted average of the computedmetrics. Numerous weighting schemes can be implemented. For example, themetrics can be weighted equally, weighted more heavily toward morerecent measurements (i.e., a “leaky” average), or weighted using otherschemes (e.g., frames located near good frames may be weighted moreheavily).

76. Various other factors can also be used in setting the thresholdvalue. For example, the set points for frames correctly and incorrectlydecoded can also be used to adjust the threshold value. The set pointscan be averaged in the manners described above. The amount of adjustmentin the threshold value can also be dependent on, for example, the numberof fingers in a rake receiver used to demodulate the signal.

77. For clarity, the invention has been described for detecting zerorate frames on the forward link. For some CDMA systems (e.g., IS-95-B),on the reverse link, the code symbols at lower rates are transmitted atfull power but pseudo-randomly transmitted in one of N possible symbollocations. For example, for eighth rate transmission, each code symbolis transmitted in one of eight possible symbol locations, with thelocation being selected by a long PN sequence. At the base stationreceiver, a selector unit selects the code symbols at the properlocations based on the hypothesized rate. Thus, the base station decoderfor the lower rates includes a selector in place of the symbolaccumulator. The decoder for the reverse link signal in an IS-2000 CDMAsystem is further described in the aforementioned IS-2000 submission.

78. For clarity, many aspects of the invention are described for aspecific implementation in a CDMA system that complies with IS-2000.However, the invention can be adopted for used with other CDMA systems.One specific CDMA system is disclosed in U.S. Pat. No. 4,901,307,entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USINGSATELLITE OR TERRESTRIAL REPEATERS” and U.S. Pat. No. 5,103,459,entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULARTELEPHONE SYSTEM.” Another specific CDMA system is described in U.S.patent application Ser. No. 08/963,386, entitled “METHOD AND APPARATUSFOR HIGH RATE PACKET DATA TRANSMISSION,” filed Nov. 3, 1997. Thesepatents and patent applications are assigned to the assignee of thepresent invention and incorporated herein by reference.

79. CDMA systems can be designed to conform to a number of currentlydefined CDMA standards, and current or future proposed standards. Forexample, the CDMA system can be designed to conform to “TIA/EIA/IS-95-AMobile Station-Base Station Compatibility Standard for Dual-ModeWideband Spread Spectrum Cellular System” or TIA/EIA/IS-98-A, -B, and -Centitled “Recommended Minimum Performance Standard for Dual-Mode SpreadSpectrum Cellular and PCS Mobile Stations,” hereinafter referred to asthe IS-95-A and IS-98 standards, respectively. CDMA systems can also bedesigned to conform to the IS-2000 or the WCDMA standards being proposedby the standards bodies ETSI and ARIB. These various CDMA standards areincorporated herein by reference.

80. The invention can also be adopted for use with other types ofcommunications systems such as time division multiple access (TDMA),frequency division multiple access (FDMA), and amplitude modulation (AM)schemes such as amplitude companded single sideband (ACSSB).

81. The foregoing description of the preferred embodiments is providedto enable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without the use of theinventive faculty. Thus, the present invention is not intended to belimited to the embodiments shown herein but is to be accorded the widestscope consistent with the principles and novel features disclosedherein.

What is claimed is:
 1. In a wireless communication system, a methodcomprising: receiving a modulated signal; demodulating the modulatedsignal to generate demodulated symbols; partitioning the demodulatedsymbols into a plurality of received frames; computing a quality metricfor a fraction of at least one of the plurality of received frames;performing a comparison of the quality metric for the fraction of aparticular received frame against a threshold value, wherein thethreshold value is selected based, in part, on the quality metrics ofreceived frames; and determining if the particular received frame a zerorate frame based on the comparison.
 2. The method of claim 1 , whereinthe quality metric relates to an energy of a received frame.
 3. Themethod of claim 2 , wherein the quality metric relates to a distancebetween the particular received frame and a codeword corresponding tothe at least one frame.
 4. The method of claim 3 , further comprising:decoding the particular received frame to form a decoded frame, thedecoded frame comprising decoded data bits.
 5. The method of claim 3 ,wherein computing the quality metric further comprises: correlating thedecoded data bits of the particular received frame with a data portionof the modulated signal.
 6. A wireless communication device performingthe method of claim 1 .
 7. A method for detecting zero rate frames in areceived data transmission, the method comprising: receiving a modulatedsignal; demodulating the modulated signal to generate demodulatedsymbols; partitioning the demodulated symbols into a plurality ofreceived frames; decoding the plurality of received frames to formdecoded frames, each of the decoded frames comprising decoded data bits;computing a quality metric for at least one of the plurality of receivedframes by correlating decoded data bits to encoded data; performing acomparison of the quality metric for a particular received frame againsta threshold value, wherein the threshold value is selected based, inpart, on the quality metrics of received frames; and determining if theparticular received frame a zero rate frame based on the comparison. 8.The method of claim 7 , wherein the threshold is based on a qualitymetric of a good frame.
 9. The method of claim 7 , wherein the thresholdis based on statistics of quality metrics of a plurality of receivedframes.
 10. The method of claim 7 , wherein computing the quality metricfurther comprises: computing the quality metric for at least a portionof a received frame.
 11. A wireless apparatus, comprising: a demodulatoradapted to demodulate received modulated signals and generatedemodulated symbols; a decoder coupled to the demodulator and adapted todecode demodulated symbols and generate decoded data bits, wherein thedemodulated symbols are partitioned into frames; and a memory storagedevice adapted for storing: a first set of computer readableinstructions adapted to calculate a quality metric of at least one frameby correlating decoded data bits to a data portion of the receivedmodulated signals; a second set of computer readable instructionsadapted to compare the quality metric to a threshold value; and a thirdset of computer readable instructions adapted to determine if the atleast one frame is an empty frame based on the comparison.